The present invention is directed to a reinforcing lamina sheet for use in multiple layer structural laminates such as reinforced beams, and to processes for forming same, and to laminates made therefrom.
Reinforced glue laminate beams, or reinforced glulam beams, are wood laminates which are reinforced with one or more synthetic layers. Glulam beams are frequently used in place of conventional wood products such as beams, columns, cantilevered supports or trusses to provide structural support and integrity in the construction industry. Glulam beams are structurally more sound and are often less expensive than conventional wood products. The synthetic reinforcement layers are typically positioned between individual wood layers of the glulam beam to improve the tensile or compressive loading of the resulting beam.
Current commercially significant glulam beams comprise layers arranged such that the synthetic layers are near both exterior surfaces of the beams, e.g., wood/reinforcing lamina sheet/wood/wood/wood/reinforcing lamina sheet/wood. The strength of the glulam beam is a function of the ability of the reinforcing lamina sheet to take load uniformly and the degree of adhesiveness between the individual lamina within the glulam beam. In this arrangement, the reinforcing lamina sheet positioned between the uppermost wood lamina and the second wood lamina is subjected to substantially pure compressive stresses when the glulam beam is used in a construction project such as a building. The reinforcing lamina sheet positioned between the lowermost lamina and the adjacent lamina is subjected to substantially pure tensile stress.
The reinforcing lamina sheets for glulam beams are typically formed of unidirectionally oriented fibers in a resin encasement that surrounds the fibers and fills the interstices between the fibers. The reinforcing lamina sheet is then selectively positioned within the glulam beam to improve the tensile or compressive properties of the beam. The reinforcing lamina sheets are typically adhered to the wood lamina by a commercial grade non-epoxy adhesive such as a resorcinol based adhesive. When the reinforcing lamina sheet comprises aramid fibers or other high strength fibers, it is often difficult to cut the final beam to a desired length or to a different width, or to finish the edges of the glulam beam.
Prior art reinforcing lamina sheets are generally formed by a fabrication process referred to as pultrusion. Pultrusion is a continuous manufacturing process for producing fiber reinforced plastic parts of indefinite length. Pultrusion involves pulling the flexible reinforcing fibers through a liquid resin bath and then densely compacting the saturated fibers through a heated die where the reinforced plastic is shaped according to the die mold and the resin cured. A drawback of this process is that the heated die step of the conventional pultrusion process subjects the composite material to significant pressure, often in the range of 50-80 psi. The surfaces of a pultruded reinforcing lamina sheet are generally smooth with little or no aramid fibers along its outermost surface. Accordingly, the surface must be abraded so that an adhesive will bond to the aramid fibers to an adjacent lamina, i.e., wood or reinforcing lamina sheet. Also, the unidirectional fibers of the pultruded reinforcing laminates are not uniformly tensioned throughout the process and, therefore, when stressed within a glulam beam, do not uniformly receive load.
An example of the prior art is U.S. Pat. Nos. 5,498,460, 5,456,781, and 5,362,545 all to Tingley. The Tingley ""545 patent describes a conventional process for forming glulam beams. Specifically, the laminate is pultruded in a straight fashion. The pultruded composite is then abraded to expose the aramid fibers in order to get a proper degree of adhesion between the wood and the composite using conventional adhesives. Once the composite is formed, the layers are glued together forming the beam and the beam is compressed using multiple xe2x80x9cC-clampsxe2x80x9d at a minimum of 100 psi with the general rule for soft woods being 125-150 psi. The beam is then cured at room temperature and allowed to sit for approximately 24 hours in its compressed state. Once the glue has hardened, the C-clamps are removed from the glulam beam and the beam is finished by trimming the sides and polished to prepare the beam for commercial sale.
The Tingley ""460 and Tingley ""781 patents are similar to the Tingley ""545 patent. The Tingley ""460 patent describes providing recesses in the laminate to facilitate adhesion between the fiber reinforced lamina and the wood layers. The Tingley ""460 patent forms these recesses by either providing a material which is nonreactive with the resin additive and removing it after the composite is formed, using a nonreactive gas or low boiling point liquid which dissipates as the resin is being cured, or abrading the composite as described in the Tingley ""545 patent. The Tingley ""781 patent also describes the use of the abrasion process and further describes the importance of covering the complete surface of the reinforcement with adhesive.
The present invention provides a reinforcing lamina sheet for forming adhesive-bonded multiple layer structural laminates, preferably glulam beams. The reinforcing lamina sheet of the invention is formed of pre-tensioned, unidirectional, i.e., substantially parallel, reinforcing filaments, preferably in the form of multifilament yarns, in combination with a resin matrix. The reinforcing lamina sheet of the invention exhibits a generally rough surface and has a relatively low density as compared to reinforcing lamina sheets prepared by conventional pultrusion processes, and can readily be glued to other sheets or substrates without special surface preparation. Preferred lamina of the invention minimize difficulties and costs associated with machining and/or trimming a completed, multilayer structural member into a finished form.
In accord with the invention, continuous filament yarns of high strength reinforcing materials, preferably aramid multifilament yarns, are formed into a unidirectional, substantially planar band which is impregnated with a thermoset resin, such as an epoxy resin, to form an impregnated band. The impregnated band of unidirectionally oriented filaments is heated on at least one side while it is maintained in a curved configuration under tension. Preferably the heating step is effected by passing the impregnated band in the length-wise direction into contact on a first side thereof with a curved hot surface, such as a large diameter roll or drum that is continuously heated by oil, to promote cross-linking and partial curing of the thermoset resin while the band is in the curved configuration. Accordingly, the impregnated band is partially cured in an arcuate, i.e., curved, configuration along the length of the band. The partially cured, arcuate band is then transported to a further heat zone which, according to various embodiments of the present invention, is advantageously either a heated chamber or another curved, heated surface such as a second oil can. Then at least the side of the band opposite the first side is heated to further promote curing and cross-linking while the band is maintained in a second configuration different from the first configuration. The second configuration can be a flat configuration or a configuration curved in a direction opposite the first configuration.
The band is uniformly tensioned throughout the process by rollers or the like. The uniform tension applied to the band in the second configuration causes at least the curved first outer surface of the band to fracture, typically by cracking, in the lengthwise direction and also in the transverse direction to a lesser extent. The fractures or cracks in the surface of the band facilitate bonding between the reinforcing lamina sheet end the wood layers of the glulam beam by allowing the adhesive used to glue the lamina sheet to the wood layers to penetrate into the lamina sheet.
In a preferred embodiment, the band is conveyed after heating and partial curing in the initial curved configuration, under substantially uniform tension through an oven or other heated chamber wherein both sides of the band are contacted with heat while the band is flexed and bent by pulling it under tension across one or more rollers. The band is then passed into contact with a second, large diameter curved heated surface which further cures the side of the band opposite the side contacted by the first heated curved surface. The substantially cured continuous laminate sheet is then wound about a mandrel with the sheet being oriented in the same direction of curvature as the first configuration, and is maintained in the curved configuration during subsequent storage and transport, prior to its use in forming a structural laminate.
In another preferred embodiment, contiguous groups of low tensile strength non-aramid filaments or yarns are incorporated in the unidirectional aramid filament band to form discreet, narrow, unidirectional portions of the band. For instance, rayon or similar fiber can be used to form selvage-like edges of he band. The non-aramid portions of the band can also be provided in one or more medial portions of the band to enhance the separation of various widths of the reinforcing lamina sheet. The selvage-like edges facilitate the finishing and trimming processes that are applied to laminates containing one or more lamina sheets of the invention because the lower tensile strength fibers forming the selvage edges are easier to grind, machine, and cut than aramid fibers.
The reinforcing filaments in the lamina sheets of the invention are substantially uniformly tensioned which in turn maximizes strength contributions to multiple layer laminates such as glulam beams, prepared using the lamina of the invention. The curvature of the lamina of the invention can enhance and improve the beam strength of a glulam beam as compared to similar beams prepared using conventional flat lamina. Additionally, although cracking normally represents a degradation in a composite product, the rough and irregular surfaces of the lamina of the invention provide laminate beams with improved shear strength by improving bonding of the lamina to other layers used to form a laminate. Preferably, the lamina of the invention comprise an epoxy resin matrix which readily bonds with high strength to adhesives typically used to form cellulosic-based laminates, such as glulam beams. Moreover, the low tensile strength fibers in the laminate decrease the cost associated with trimming the completed beam.